The broadest goal of the research described in this proposal is to increase understanding of how human brightness and suprathreshold contrast sensation depend upon the spatial (i.e., size and/or shape) and intensive (i.e., luminance and/or contrast) parameters of the context within which visual stimuli are viewed. To this end, a set of six interrelated studies is described whose aims include the formulation, evaluation and refinement, using an empirical/inductive approach to psychophysical experimentation in combination with exploratory computer simulations, of a heuristic model for the grating induction effect of McCourt (1982). Computer simulations are described which employ an isotropic, two-dimensional implementation of a multiple mechanism difference-of-Gaussian (DOG) filter model similar to those of Wilson & Bergen (1979) and Moulden & Kingdom (1991). Data obtained from contrast matching experiments are presented which suggest that a completely linear version of this model is capable of accounting for certain structural (i.e., form-related) aspects of the grating induction effect. Additional evidence from direct brightness matching studies is described, however, which indicates that the intensive features of grating induction will require the inclusion of one or more nonlinear processing stages, and experiments are proposed to infer the form and/or processing loci of the nonlinear transduction process(es) which provide(s) the most satisfactory account of grating induction data. A second goal of the proposed research is to further elucidate the relationship between grating induction and two other robust suprathreshold contrast effects: the contrast-contrast effect of Chubb, Sperling & Solomon (1989), and the high-on-low spatial frequency inhibition of grating induction (McCourt & Foley, 1985). A series of experiments is described whose goal is to measure and compare the summation regions and spatial frequency dependence of these three effects.